Left ventricular pulsus alternans, arhythmic beat to beat variation in left ventricular systolic pressure has long been considered a sign of myocardial disease, commonly occurring in the setting of increased afterload such as aortic stenosis and hypertension, especially in the setting of myocardial failure. We noted the occurrence of left ventricular pulsus alternans, arrhythmic beat to beat variation left ventricular systolic pressure and outflow gradient in 35 of 200 consecutive patients with hypertrophic cardiomyopathy undergoing hemodynamic studies and in sinus rhythm. All patients with left ventricular pulses alternans had severe outflow gradients at rest or during provocation. No patient without obstruction at rest or provocation demonstrated left ventricular pulsus alternans. Eight patients with severe resting outflow obstruction and left ventricular pulsus alternans underwent ventricular septal myotomy/myectomy; all had successful abolition of basal outflow gradient and none demonstrated left ventricular pulsus alternans during post-operative hemodynamic study. Thus, left ventricular pulsus alternans is commonly seen in patients with hypertrophic cardiomyopathy with severe left ventricular outflow gradients, and may represent inadequate left ventricular contractile function in the presence of high left ventricular systolic pressures.